The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) apparatus and method for providing energy to biological tissue, and more particularly, to a radio frequency (xe2x80x9cRFxe2x80x9d) ablation apparatus for controlling the flow of current through a biological site so that the volume of ablation lesions may be controlled.
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed by percutaneous ablation, a procedure in which a catheter is percutaneously introduced into the patient and directed through an artery to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat or at least an improved heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system.
There are two general methods of applying RF energy to cardiac tissue, unipolar and bipolar. In the unipolar method a large surface area electrode; e.g., a backplate, is placed on the chest, back or other external location of the patient to serve as a return. The backplate completes an electrical circuit with one or more electrodes that are introduced into the heart, usually via a catheter, and placed in intimate contact with the aberrant conductive tissue. In the bipolar method, electrodes introduced into the heart have different potentials and complete an electrical circuit between themselves. In the bipolar method, the flux traveling between the two electrodes of the catheter enters the tissue to cause ablation.
During ablation, the electrodes are placed in intimate contact with the target endocardial tissue. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C.
A basic configuration of an ablation catheter for applying RF energy includes a distal tip which is fitted with an electrode device. The electrode device is the source of an electrical signal that causes heating of the contacting and neighboring tissue. In the unipolar method, the electrode device may include a single electrode used for emitting RF energy. This single electrode acts as one electrical pole. The other electrical pole is formed by the backplate in contact with a patient""s external body part. A RF source is applied to the electrode. The RF source is typically in the 500 kHz region and produces a sinusoidal voltage. When this is delivered between the distal tip of a standard electrode catheter and a backplate, it produces a localized RF heating effect and produces a well defined, deep acute lesion slightly larger than the tip electrode.
In some procedures a lesion having a larger surface area than that produced by a single electrode in a unipolar arrangement may be required. To this end numerous ablation catheters have been designed. In one catheter designed to provide a larger surface ablation area, an electrode device having four peripheral electrodes which extend from a retracted mode is used. See U.S. Pat. No. 5,500,011 to Desai. When extended, i.e., fanned out, the four peripheral electrodes and the central electrode form an electrode array that covers a larger surface area of the tissue than a single electrode. When used with a conventional RF power source, and in conjunction with a backplate, the five electrodes produce five lesion spots distributed over the area spanned by the electrode array. The lesions produced are discontinuous in relation to each other and there are areas between the electrodes that remain unablated. This device must be manipulated so that when expanded, all electrodes are in contact with the endocardium. An xe2x80x9cend onxe2x80x9d approach is required such that the end of the catheter, on which all five electrodes are mounted, is in intimate contact with the target tissue.
In another catheter an electrode device having a central electrode and a number of peripheral electrodes which also fan out from a retracted mode is used. During ablation a backplate is not used; instead the central electrode functions as the reference while the peripheral electrodes have multi-phase RF power applied to them. For example, see U.S. Pat. No. 5,383,917 to Desai et al. While this technique provides a more continuous lesion covering a larger surface area of the tissue, the ablation volume is relatively shallow with a nonuniform depth of the lesion. This arrangement also requires the same manipulation of the catheter such that an end-on contact is made by the expanded electrodes, as discussed above. Lesions having a non-uniform ablation volume are undesirable as the depth at one part of the lesion may not be sufficient to stop the irregular signal pathways. Arrhythmia may reoccur because the irregular signals may pass under such an ablation volume and the procedure must then be repeated to once again attempt to obtain an ablation volume having sufficient depth.
The mechanical configuration of both of the above-described techniques comprises an expanding approach. When used for ablation, an electrode device is typically part of a catheter system. Accordingly, it is desirable to minimize the diameter of the electrode device during introduction to and withdrawal from the patient to lessen trauma to the patient. Therefore, electrode devices having peripheral expandable electrodes must be configured so that the peripheral electrodes are expandable to a large size yet are retractable to as small a size as practical. Such requirements pose design and manufacturing difficulties due to the movement of mechanical parts required for proper operation. Further considerations are the undesirable complexity and increased manufacturing cost associated with an expandable a catheter.
Hence, those skilled in the art have recognized a need for a structurally stable invasive ablation apparatus and method that are capable of controlling the flow of current through a biological site so that lesions with controllable surface and depth characteristics may be produced and the ablation volume thereby controlled. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to an apparatus and a method for controlling the application of energy to a biological site during ablation to thereby control the surface area, the continuity, and the depth of lesion produced during ablation.
In one aspect, the invention is directed to an apparatus for delivering energy to a biological site comprising a catheter having a plurality of electrodes at its distal end. The distal end is positionable so that the electrodes are located at the biological site. The apparatus also includes a backplate positionable proximal the biological site so that the biological site is interposed between the electrodes and the backplate and a power control system providing power to each of the electrodes, the power having a duty cycle with an on period and an off period. During the on period of the duty cycle, the power is selected such that at least two electrodes have voltage levels that differ from each other and at least one electrode has a voltage level that differs from the backplate so that current flows between the two electrodes and between at least one electrode and the backplate. During the off period of the duty cycle, the power is selected such that each electrode and the backplate have substantially the same voltage levels so that substantially no current flows between the electrodes and between the electrodes and the backplate.
In more detailed aspects, the power control system provides separate power to each of the plurality of electrodes with the power to each electrode being individually controllable as to duty cycle. In another detailed aspect, the power control system controls the duty cycle of the power to be approximately ten percent. In a further detailed aspect, a temperature sensing device is located at least one of the electrodes for providing a temperature signal to the power control system representative of the temperature at the electrode. The power control system controls the duty cycle of the power in response to the temperature signal. In another detailed aspect, the apparatus includes a measurement device that senses at least one characteristic of the power applied to at least one electrode and provides a power measurement signal and the power control system receives the power measurement signal and determines an impedance measurement based on the power measurement signal and controls the duty cycle of the power in response to the power measurement signal.
In another aspect, an apparatus for delivering energy to heart tissue comprises a catheter having at least three electrodes arranged in a linear array at its distal end. The distal end is positionable so that the electrodes are located at the heart tissue. The apparatus also includes a backplate positionable so that the heart tissue is interposed between the electrodes and the backplate, a power control system providing power to each of the electrodes, the power having a duty cycle with an on period and an off period. During the on period of the duty cycle, at least two of the electrodes have different phase angles and the power applied to at least one electrode has a voltage level that differs from the backplate so that current flows between said two electrodes and at least one electrode and the backplate. During the off period of the duty cycle, the power is selected such that each electrode and the backplate have substantially the same voltage levels so that substantially no current flows between the electrodes and between the electrodes and the backplate.
In yet another aspect, a method for delivering energy to a biological site comprises the steps of positioning a catheter having a plurality of electrodes at its distal end at the biological site; positioning a backplate proximal the biological site so that the biological site is interposed between the electrode device and the backplate; and providing power to each of the electrodes, the power having a duty cycle with an on period and an off period. The method also includes the steps of, during the on period of the duty cycle, selecting the power such that at least two electrodes have voltage levels that differ from each other and at least one electrode has a voltage level that differs from the backplate so that current flows between the two electrodes and between at least one electrode and the backplate; and during the off period of the duty cycle, terminating the power such that substantially no current flows between the electrodes and between the electrodes and the backplate.
These and other aspects and advantages of the present invention will become apparent from the following more detailed description, when taken in conjunction with the accompanying drawings which illustrate, by way of example, the preferred embodiments of the invention.